1. Field of the Invention
The present invention relates to a tool for press bending (hereinafter, referred to as a “bending tool”) mounted on a press bending machine (or a “press brake”) which is a bending apparatus for bending mechanical parts, such as metal plates. More particularly, the invention relates to a lower die of the bending tool.
2. Description of the Related Art
Many L-shaped or U-shaped metal pieces are manufactured as brackets or reinforcing members of various machines or devices. A related art bending tool for manufacturing such L-shaped or U-shaped metal pieces and operation states thereof are described in JP-B-58-41927 and illustrated in FIG. 9 of the present application.
FIG. 9 illustrates the manufacture of an ordinary L-shaped metal piece. With this unit, an L-shaped bending tool (i.e., a die) 2 and a V-block 3 are disposed opposite to each other along a vertical direction. The die 2 corresponds to an upper die fixed to a bending machine body 1 with a bolt 5 or other means. The V-block 3 corresponds to a lower die fixed to a table 4 of a bending machine. One or both of the die 2 and the V-block 3 are moved upward or downward to approach their counterpart for the bending operation. The die 2 has a substantially right-angled, wedge-shaped sharp tip of which bisector substantially corresponds to a plumb line. The V-block 3 has a groove, i.e., a recess, consisting of sloped surfaces crossing each other at the right angle. The V-block 3 also has a bisector which substantially corresponds to the plumb line. The die 2 and the V-block 3 are configured such that the tip of the die 2 and the groove of the V-block 3 conform to each other without any misalignment when the die 2 and the V-block 3 are brought together. When the die 2 is pressed against the V-block 3 on which a metal sheet 6 is placed, the metal sheet 6 is pressed by the die 2 into the groove of the V-block 3. Pressing pressure causes a plastic deformation of the metal sheet 6 into an L shape of which bending portion being substantially right-angled. In this manner, an L-shaped metal piece is manufactured.
A U-shaped metal piece is manufactured in the following manner. The metal sheet 6 is placed on the V-block 3 in a manner similar to that described above. The metal sheet 6 is bent in a first bending operation into L shape at a certain bending position (i.e., location) set for the bending of the metal sheet 6 into U shape. In this manner, an L-shaped work 6a (indicated by dashed dotted lines in FIG. 9) bent into the right angle at a first bending point 7a is obtained. The thus-obtained L-shaped work is moved along a front-rear direction on the V-block 3 to set a bending position (i.e., location) for bending into U shape (in FIG. 9, the right side is defined as a front side and the left side is defined as a rear side, which definition of the direction will be adopted throughout the specification.). The metal sheet 6 is bent at the bending position into L shape. In this manner, a U-shaped metal piece 6b (indicated by solid lines in FIG. 9) is obtained which is bent into the right angle at a second bending point 7b and is also bent at the first bending point 7a. As described above, the U-shaped metal piece 6b is manufactured through bending operations into L shape at two different positions on the metal sheet 6. A side between the two positions at which the metal sheet 6 is bent into L shape is defined as a base 8a of the U-shaped metal piece 6b. 
Although the related art bending tool has certain effects and utility as described above, it has the following deficiencies. When the metal sheet 6 is bent into L shape as illustrated in FIG. 9 using the above-described related art bending tool, the metal sheet 6 placed on the block 3 at a horizontal position is bent into the right angle at the bending point when the die 2 and the block 3 are brought together and thereby the metal sheet 6 is moved upward by 45 degrees at a front side of the bending machine body 1. The upward movement of the metal sheet 6 is unsafe to an operator who puts materials in and out at this front position.
The metal sheet 6 is bent into L shape twice using the related art bending tool during the manufacture of the U-shaped metal piece 6b as illustrated in FIG. 9. When the metal sheet 6 is bent in the second bending operation into L shape, the base 8a of the U-shaped metal piece 6b extends at 45 degrees with respect to the horizontal plane from the second bending point 7b formed in the second bending operation. A vertical side 8b (which corresponds to one of legs of the U-shaped metal piece 6b) extends at 90 degrees from an end of the base 8a (i.e., from the first bending point 7a). Thus the vertical side 8b extends from the end of the base 8a at 135 degrees (i.e., at a reversed angle of 45 degrees). Even if the vertical side 8b extends at the reversed angle of 45 degrees, there is no problem as long as a required length of the vertical side 8b is sufficiently small. However, when it is desirable to manufacture a U-shaped metal piece 6b with a longer vertical side 8b (i.e., a U-shaped metal piece 6b of larger depth with respect to the length of the base 8a), an end 8c of the vertical side 8b interferes with a surface of the bending machine body 1 as illustrated in FIG. 9. It is therefore impossible to provide any longer vertical side 8b. This means that the related art bending tool can provide only U-shaped metal pieces 6b of comparatively small depth with respect to the length of the base 8a. In the exemplary U-shaped metal piece 6b illustrated in FIG. 9, a dimensional ratio of the base to the vertical side is about 1:0.55.
In order to solve these related art problems, an object of the invention is to achieve a bending tool which provides a U-shaped metal piece of increased depth, reliably bends a work which is longer (as compared with a processing material of ordinary length) without interference with a processing machine or a neighborhood thereof, and provides a bending operation safe to an operator involved in the bending operation.